In pneumatic tires as a typical example of the rubber article, it is attempted to mainly conduct the reinforcement with steel cords by applying a rubberized steel cord made by twisting a plurality of brass-plated steel filaments or made of a single steel filament to a belt or a carcass. In order to utilize the steel cord as a reinforcing member for the tire, it is required to surely adhere the steel cord to a coating rubber, and hence a peripheral face of a filament constituting the steel cord is subjected to a brass plating.
As to the brass plating, it is examined to adjust a ratio of copper to zinc in the brass, a plated thickness and the like for ensuring the adhesion property to rubber, and hence a fixed knowledge therefor is established.
By subjecting the filament constituting the steel cord to an adjusted brass plating based on the above knowledge is improved the adhesion property to rubber. Nevertheless, various conditions are required to rubber to be adhered. For example, it is required to ensure a sufficient adhesion force by an adhesion rate between cord and rubber or a complete bonding therebetween for curing and building a tire in a certain time. That is, a so-called initial adhesion property is required, so that it is necessary to add a sufficient ratio of Co salt or Ni salt as an adhesion promoter to rubber or to compound sulfur at a higher ratio.
The thus added adhesion promoter including sulfur is effective to promote the adhesion reaction, but causes the oozing of the adhesion promoter from the uncured rubber or so-called blooming of chemicals. As a result, the operability in the plying of the uncured rubber sheet at, for example, a tire building step lowers and also the closing property or adhesion property between the uncured rubber sheet and rubber is obstructed and further a residue of the adhesion promoter in a cured rubber causes a cutting reaction of rubber molecule or a reversion, which result in the lowering of the tire durability.
From a viewpoint of preventing the occurrence of such problems, it is attempted to improve the adhesion property between a rubber composition containing no adhesion promoter and a cord by changing a kind of the adhesion promoter, particularly a kind of an acid in Co salt or Ni salt, or by locating the adhesion promoter (cobalt metal salt) as a thin film between the coating rubber and the cord for decreasing the adhesion promoter in the coating rubber to be adhered with the steel cord. The latter technique is disclosed in, for example, JP-A-10-324753.
Incidentally, since the adhesion promoters, particularly adhesion promoters such as cobalt metal salt and the like are expensive, the decrease of the adhesion promoter in the coating rubber is effective to not only improve the performances of the tire but also reduce the compounding cost of rubber and is important from a viewpoint of source-saving.
However, the change in the kind of the adhesion promoter is only the application of a local optimization. Since the Co content is obliged to be basically the same, it is unavoidable to cause two-conflicting condition that when the initial adhesion property is improved, the adhesion durability lowers or the blooming property lowers.
On the other hand, the method of locating the adhesion promoter as a thin film between the coating rubber and the cord as disclosed in JP-A-10-324753 is possible to certainly remove the compounding of Co in the coating rubber. However, a ratio of Co diffused in the coating rubber becomes inversely large before the adhesion reaction, so that it is required to arrange the thin film containing the adhesion promoter at a thickness of about several ten μm and hence it can not be said that the effect of decreasing Co is sufficient and a further improvement is desired.
As to the adhesion between the coating rubber and the steel cord made of brass-plated filament(s), there should be considered various requirements that not only the initial adhesion property is excellent but also there is not caused a problem resulted from the deterioration of rubber material including an adhesion boundary face between cord and rubber when the tire is placed in a deterioration environment during the use, and further the operability at the tire production step is improved and the compounding cost is controlled and the like.
As mentioned above, it is a first principal requirement that the initial adhesion property is excellent as regards the adhesion property between the steel cord made of brass-plated filament(s) and the rubber coating it. For this end, it is examined to control the brass plating properties on the surface of the filament, and there are reported various examination results on a plating composition, particularly a plating composition on an outermost surface, a plated thickness, an influence of an oxidation degree of copper and zinc and the like. Also, there are proposals on the reduction of a phosphoric acid coating layer produced in the drawing to exist on the outermost surface and contributing to ensure the drawability.
However, the steel wire is required to take a work strengthening by repeatedly subjecting a wire rod of, for example, about 5 mm to a drawing, so that the change in the plating influencing upon the drawing is naturally restricted. In fact, a surface brass-plated layer applied to the wire and conducting an adhesion to rubber is largely different from a matrix portion to be plated or the wire rod in the chemical composition. For instance, the ratio of Cu and Zn contents is reversed, or the plated surface is frequently covered with a phosphoric acid coating or zinc oxide to suppress the activity of the plated layer itself.
Furthermore, when the tire is placed in the deterioration environment during the use, it is important to cause no problem resulted from the deterioration of the rubber material including the adhesion boundary between cord and rubber as previously mentioned. In this connection, the conventional adhesion promoter containing sulfur is effective for promoting the adhesion reaction, but does not contribute to control the adhesion when the rubber-cord composite in the tire or the like is exposed to water or oxygen (or active gas in air) under a heat environment and may promote the adhesion deterioration according to circumstances. Therefore, when the tire and the like are used under a thermally severe environment co-existing a relatively large amount of water and air, e.g. a high-temperature and high-humidity zone such as subtropical zones of Southeast Asia, it is important to improve so-called adhesion properties after the deterioration such as adhesion property in the resistance to moisture heat and the like in addition to the improvement of the initial adhesion property. However, it is difficult to simultaneously establish the initial adhesion property and the adhesion properties after the deterioration.
In order to solve these problems on the adhesion property, JP-A-6-49783 proposes a technique focusing attention on the amount of phosphoric acid at the surface of the wire constituting the steel cord.
However, this publication discloses that as the amount of phosphoric acid remaining on the surface of the wire becomes large, the adhesion properties after the deterioration are excellent under a high-temperature and high-humidity environment but it is difficult to ensure the initial adhesion property, while as the remaining phosphoric acid becomes small, the initial adhesion property is improved but the adhesion properties after the deterioration become insufficient. For this end, it is generally recognized that it is difficult to simultaneously establish both the properties.